This invention relates generally to crystal controlled oscillators and more particularly to the suppression of adverse effects on the output of the oscillator due to vibration of the crystal resonator.
When a piezoelectric material such as a quartz crystal resonator is subjected to a stress, a shift of its natural resonant frequency occurs. If, for example, the stress applied is periodic, such as when a crystal resonator is vibrated, the resonant frequency shifts in a similarly periodic manner. This frequency shift gives rise to undesired frequency modulated sidebands.
In electronic systems utilized for modern communications, navigation, and radar, a relatively high spectral purity is required and frequency modulated sidebands produced as a result of vibration cause severe adverse effects to occur. For example, under certain vibration and frequency multiplication conditions, the desired carrier frequency can disappear entirely leaving only the undesired sidebands. Also, the signal to noise ratio can reach a level where catastrophic effects result, particularly in coherent radar sets resulting in a reduction of the probability of detection of echoes in such systems.
By substantially suppressing, or entirely eliminating the vibration-induced sidebands, none of the above mentioned adverse effects occur. Both passive and active approaches have been taken heretofore to reduce effects of vibration on crystal resonators. The proper circuit design with compensating networks coupled to the resonator comprises one passive means while another approach consists in utilizing two resonators in series. An active method has been proposed by J. M. Przyjemski in a publication entitled, "A Compensation Technique For Acceleration-Induced Frequency Changes in Crystal Oscillators", which appeared in the Proceedings of NAECON 78, May, 1978. What is suggested in this publication is sensing the components of acceleration normal to the plane of zero g-sensitivity by an accelerometer which produces a correction voltage which is applied to the frequency control input of the crystal oscillator. Such compensation is accomplished by adjusting the resonant frequency of the oscillator but not the crystal resonator. Another approach uses a varactor diode to alter the frequency of the crystal resonator by changing the load capacitance. This is suggested in a publication entitled, "Acceleration Sensitivity Compensation In High Performance Crystal Oscillators" by Donald A. Emmons, which comprises Application Note Number 61 of the PTTI Meeting, November, 1978. In such an application, however, the nonlinearity of the varactors used for the implementation of the method suggested limits the ultimate amount of reduction possible.
It is an object of the present invention therefore to provide an improvement in the suppression of vibration induced effects on crystal controlled oscillators.
It is another object of the present invention to provide for suppression of vibration-induced sidebands in the output of crystal controlled oscillators.
And yet a further object of the present invention is to provide a direct compensation to a crystal resonator for eliminating vibration induced sidebands in the output of a crystal controlled oscillator.